1. Field of the Invention
The present invention relates to the field of electronic circuit fabrication.
2. Prior Art
The inventor's primary interest in the present invention relates to the fabrication of high density multichip interconnects (HDMIs), devices which typically receive a plurality of integrated circuits in unpackaged chip form and provides interconnects therefor so as to provide the functional equivalent of VLSI and wafer scale integration without the attendant problems thereof. Since similar processes are currently used and very similar problems are also encountered in the fabrication of integrated circuits themselves, the prior art relating to HDMI and integrated circuit fabrication will be described. It should be understood, however, that while the present invention is particularly useful in these two areas of electronics, the same is not necessarily so restricted.
In the fabrication of HDMIs and in semiconductor device manufacturing, typical processing includes the steps of depositing a metal layer and then patterning the same, either for providing an interconnecting layer in HDMIs and semiconductors, or in the case of HDMIs, for also providing a metal mask through which one or more polymer layers therebelow may be plasma etched. For this purpose, various techniques are well-known for patterning such metal layers, one of the most common being through the use of a contact printing process. In accordance with this process, the metal layer is coated with a photo resist, typically using spin-coating techniques, and then exposed through an appropriate mask laid over the semiconductor wafer. Thereafter, either the exposed or the unexposed portion of the photo resist, depending upon the type of photo resist used, is dissolved away, with the metal layer then being chemically etched through the openings in the photo resist to define the desired pattern. Such process provides good accuracy and is relatively easily carried out with standard equipment. However, damage to the photo resist layer and/or the mask can result when the mask is aligned as desired with respect to the wafer by the mask aligner prior to exposure of the photo resist layer, which damage may result in the fabrication of a faulty HDMI or semiconductor, or in the case of damage to the mask, subsequently produced products also until the mask is replaced.
Other techniques have also been used, such as by way of example, a very similar process wherein the mask is spaced safely away from the photo resist. These processes tend to eliminate the physical damage problem, though are less accurate than the contact printing process because of the severe limitations they impose on the exposure radiation. Still other non-contact exposure techniques are also known, though they too have similar strict requirements and limitations.
One technique that has been used, particularly in the fabrication of HDMIs, is to coat the metal layer with a relatively thick layer of polyimide, again typically by spin coating, and to remove the polyimide in the desired pattern by laser to expose local regions of the metal layer therebelow for subsequent chemical etching. Depending upon the process, it may be desired to remove one or more layers of polyimide therebelow also, in which case after the metal layer is patterned as described, the polyimide regions exposed therebelow may be plasma etched to remove the polyimide in the desired pattern. In any event, the portions of the polyimide layer not removed by the laser must provide a high integrity mask for the subsequent chemical etching of the metal layer, as the slightest pinhole in the polyimide may result in a pinhole in the metal layer therebelow, resulting in the plasma etching of a pinhole in the polyimide layer under the metal layer, ultimately resulting in a short between metal layers when a subsequent metal layer is deposited thereover. To provide the required integrity in the polymer layer as a masking layer, the polymer layer must be relatively thick, typically being on the order of ten microns thick. While such layers may be patterned by lasers as desired, the process is time consuming and relatively expensive, as the laser must be repetitively pulsed at each point (pixel) in the pattern a relatively large number of times to reliably remove the polyimide layer at that location. Further, increasing the power of the laser has little effect on the material removal rate because the opaqueness of the polymer to the laser light confines the material removal to the surface region thereof anyway. Also, because the laser creates such straight and sharp edges in the openings created through the thick polyimide layer, a subsequent metallization layer will have very poor coverage at the resulting top sharp edges and bottom corners. In comparison, plasma etching yields a tapered wall, giving a much more uniform thickness in the subsequent metallization layer. Also, the sharp corners left by the laser subsequently result in poor photoresist coverage at the sharp corners, thereby causing early breakdown when etching.
Also well known in the prior art are vapor deposition techniques for depositing layers of various materials. See for instance U.S. Pat. Nos. 3,342,754 and 3,288,728. One such technique comprises the heating of a monomer in a vacuum chamber to boil off the monomer, with the same depositing as a polymer onto the surface of the desired articles in the same vacuum chamber. This process, by way of example, has been used to protect finished printed circuit boards against water absorption by the deposition of polymers on the order of 0.001 to 0.002 inches thick.